In web centre control, sensors are arranged on each side of a web to detect the edges of the web. Two sensors may be adjusted synchronously with respect to one another in opposite directions using a single-motor sensor adjustment device with threaded spindles running in opposite directions, to guide the centre of the web of material essentially to the centre of the machine. Displacement of the web of material or set value adjustment can only be performed in the measuring range of the sensors. The measuring range of the sensors is preset for example in the case of a circular, optical sensor by the diameter of the sensor. If a new web is drawn into the machine, the two sensors which are moved synchronously guide the web of material back onto the centre of the machine, after an edge search, irrespective of the position in which the web of material was drawn into the machine. After a tear in the web, the newly drawn-in web of material can thus be roughly prepositioned. Then, after an edge search, the sensors are moved synchronously to guide the web of material back to the centre of the machine.
If two independently adjustable sensors are provided for web centre control, a web of material can be controlled over the entire adjustment range of the two sensor adjustment devices, but the web of material must be drawn into the machine as accurately as possible in the position in which it is to be subsequently controlled. After a tear in the web or after a new edge search by the sensors, the previously set position of the web of material is lost so that repositioning is necessary.